1. Field of the Invention
The present invention relates to an error correction method and a transmission apparatus, and in particular to an error correction method and a transmission apparatus applied to a transmission using a frame which comprises a header including control data and a payload including actual data.
2. Description of the Related Art
In North America, SONET (Synchronous Optical Network), which is standardized by ANSI (American National Standards Institute), is used as an optical synchronous transmission network. SONET uses OC-1 (Optical Carrier level 1) having a bit rate of 51.48 bits/second as a basic communication module. A transmission rate which is equal to 51.48 bits/second multiplied by n is represented by OC-n. At present, transmission rates up to OC-12 are standardized. A frame of OC-3 has the same structure as STM-1 (Synchronous Transport Module-1) of SDH (Synchronous Digital Hierarchy).
Generally, an error occurs in a transmission. In STM-1, for example, an occurrence of an error is monitored based on B1 and B2 bytes contained in a section overhead (SOH) having a size of nine bytes by nine rows, as shown in FIG. 1.
The B1 byte, for example, indicates BIP (Bit Interleave Parity)-8 and is used for detecting a transmission error. A value of the B1 byte is calculated as follows. That is, data to be transmitted is divided into groups, each of which consists of eight bits. A parity of the first bits of the respective groups is inserted in the first bit of the B1 byte, a parity of the second bits of the respective groups is inserted in the second bit of the B1 byte, and a parity of the third bits of the respective groups is inserted in the third bit of the B1 byte. The same process is performed for all eight bits of the groups. The B2 bytes indicate BIP-Nxc3x9724 and are calculated in a similar manner for Nxc3x9724 groups. It should be noted that values of the B1 and B2 bytes of a frame are obtained based on the data of an immediately succeeding frame. Additionally, the parity of the BIP-8 and BIP-Nxc3x9724 is an even parity.
According to the conventional function of detecting an error in SONET or SDH, since only an occurrence of an error can be detected, the error cannot be corrected when a transmission quality is degraded. For this reason, conventionally, when a transmission quality is degraded, a transmission channel is switched to another one.
In a market of optical transmission systems in North America, IXC (Inter Exchange Carrier: a generic name of long-distance telephone companies such as ATandT and MCI) has become widespread overtaking the conventional BOC (Bell Operating Company). Thus, the IXC market of a long-distance transmission has become more important. However, the conventional function of detecting a transmission error in SONET or SDH has a problem that the transmission line is frequently switched due to a degraded quality of transmission since degradation of amplifiers provided in the transmission line cannot be ignored in a long-distance transmission.
In order for a receiver to test an error-correcting function, it is necessary to generate an error during a transmission. FIG. 2 is a diagram showing an example of a system which can test an FEC (forward error correction) function by using an attenuator. A transmitter FEC 2 is supplied with a signal from a pulse generator PPG (pulse pattern generator) 1 which comprises a PN (pseudonoise) generator circuit. Then, the transmitter FEC 2 generates and transmits a signal which includes an FEC error-correcting code using the signal supplied from the pulse generator PPG 1 as a main signal. An attenuator 3 provided in a transmission line attenuates the transmitted signal. A receiver FEC 4 receives the attenuated signal and corrects errors generated in the signal. According to this system, it is possible to check an improvement of an error rate achieved by using the FEC error-correcting code.
FIG. 3 is a diagram more specifically showing the system shown in FIG. 2. A transmitter 58 adds FEC check bits (51) to the signal from the pulse generator PPG 1 and performs a B2 calculation (53). The calculated values are written in the B2 bytes of an overhead of the signal to be transmitted. The attenuator 3 attenuates the signal, and a receiver 59 receives the attenuated signal. An FEC 54 corrects errors of the received signal, and an error counter 56 counts a number of the corrected errors. Additionally, the receiver 59 checks the B2 bytes (55) and determines an error rate of the channel (57), and thus confirms an improvement of the error rate of the transmission.
As mentioned above, the receiver 59 evaluates an improvement of the error rate achieved by the FEC by checking B2 parity. However, generation of an error can only be controlled on a basis of an average rate in the transmission test using the attenuator 3. Thus, when an error of an odd number of (three or more) bits is generated, the FEC using an even parity may erroneously correct the error. In this case, an error detector 5 detects a main signal error.
In other words, according to the system shown in FIG. 3, since the performance of the FEC is tested based on an improvement of the average error rate by the receiver 59 when the FEC is performed, it is difficult to test whether the error correction is properly performed as designed.
Accordingly, it is a first object of the present invention to provide an error correction method which can easily test an error-correcting function of a transmitter and a receiver in a transmission which uses a frame including a header and a payload.
It is a second object of the present invention to provide a transmission apparatus which can easily test an error-correcting function of the transmission apparatus in a transmission which uses a frame including a header and a payload.
It is a third object of the present invention to provide an error correction method which can easily avoid a mismatch of a state of validation or invalidation of an error-correcting function between a transmitter and a receiver.
It is a fourth object of the present invention to provide a transmission apparatus which can easily avoid a mismatch of a state of validation or invalidation of the error-correcting function between a transmitter and a receiver.
The first of object of the present invention can be achieved by an error correction method in a communication using a frame which comprises a header including control data and a payload including actual data, the method comprising:
a pseudo error inserting step of inserting a pseudo error in a check bit of the frame in a transmitter.
The second object of the present invention can be achieved by a transmission apparatus which performs a communication using a frame which comprises a header including control data and a payload including actual data, the apparatus comprising:
a pseudo error inserting part which inserts a pseudo error in check bits of the frame to be transmitted.
In these inventions, since the pseudo error is inserted in the check bits of the frame to be transmitted, the error-correcting function can be easily tested.
The third object of the present invention can be achieved by an error correction method in a communication using a frame which comprises a header including control data and a payload including actual data, the method comprising:
a step of inserting information on a state of validation or invalidation of an error-correcting function of a transmitter in an unused area of the header, which information indicating whether or not an error correction is performed in the transmitter.
The fourth object of the present invention can be achieved by a transmission apparatus which performs a communication using a frame which comprises a header including control data and a payload including actual data, the apparatus comprising:
an information inserting part which inserts information on a state of validation or invalidation of an error correcting function in an unused area of the header of the frame to be transmitted, the information indicating whether or not an error correction is performed.
In these inventions, since the information on whether or not an error correction is performed is inserted in an unused area of the header of the frame to be transmitted, a receiver can detect a state of validation or invalidation of the error-correcting function of the transmitter. Thus, it is possible to easily avoid a mismatch of the state of validation or invalidation of the error-correcting function between the transmitter and the receiver.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.